Prosociality refers to the behavior of one animal that benefits another. The influence of the strength or weakness of the affiliative relationship between partners on prosociality remains poorly understood. We tested brown capuchin monkeys’ tendency to behave prosocially during a food-sharing task in which each individual was paired with a socially strong partner and socially weak partner. Social index scores, which are based upon the duration of affiliative behavior between individuals, were used to determine socially strong and socially weak dyads. During testing, one subject was secluded in a compartment containing food, and a partner was confined to an adjacent compartment. Subjects could choose to open an interconnecting door by removing a lock, which could only be accessed by the subject. If the subject opened the door, the partner could enter the subject’s compartment and access the food. Of four adult females, two females opened the door more often than expected for a socially strong partner and refrained from opening the door more often than expected for a socially weak partner, Subject 1’s chi-square(1,24)= 8.0, p=.005; Subject 2’s chi-square(1,24)=20.3, p<.001. Results provide evidence that stronger affiliative relationships are associated with increases in prosocial behavior. Social index scores are a quantitative measurement of relationship strength and may provide more accurate assessment of the influence of social dynamics on prosociality than qualitative measures such as kinship.